We have studied the fuel behavior in Port-injection gasoline engines as the following: 1. We have developed a 100%-sampling quantitative analysis method where fuel is sealed up in the intake port and cylinder at a specific point during firing operation, using an engine with intake and exhaust valves that are opened and closed by electronic control. 2. As a result of our analysis of steady and transient state characteristics of fuel behavior using this method, it was verified that the amount of wall-wetting fuel in the port and cylinder is apparently different before and after the warm-up process. As for transient fuel behavior, a delay in fuel transfer has been acknowledged in the amount of wall-wetting fuel not only in the port but also in the cylinder. Different from the existing indirect analysis, this method enables direct measurement of fuel behavior even during the actual firing operation. This allows the separation of the amount of fuel behavior between the intake port and the cylinder to occur, thus making it clear that the amount of wall-wetting fuel not only in the port but also in the cylinder should be taken into consideration when evaluating the fuel transfer delay in the transient performance. By using this method, we were able to gain a complete picture of the fuel behavior, which has always been rather ambiguously defined.